The present invention relates to a two color electrostatographic or electrostatic copying process.
A novel and unique two color electrostatic copying process is disclosed in copending U.S. patent application Ser. No. 912,273, filed June 5, 1978, now U.S. Pat. No. 4,250,239 entitled "COLOR ELECTROSTATOGRAPHIC PROCESS AND MATERIAL FOR PRACTICING SAME," which is assigned to the same assignee as this application. The present invention constitutes improvements to the basic copying process which yet further improve the quality of two color copies produced thereby.
Color electrostatic copying machines which produce full color copies are known in the art. These are generally of two types. The first type comprises a single photoconductive drum or belt which is exposed to a light image of an original document three times through filters of three primary colors respectively. After each imaging operation, a toner substance of a corresponding color is applied to the drum to form a color toner image which is transferred to a copy sheet. In this manner, three color toner images are sequentially formed on the drum and transferred to the copy sheet in register to produce a color copy. Often, a fourth black toner image is formed and transferred to the copy sheet in register with the three color toner images.
In such a copy machine it is essential that the toner images be transferred to the copy sheet in perfect register. The control mechanism for such a copying machine is therefore intricate and expensive. The three or four imaging operations for each copy require a disproportionate amount of time, making the process very slow.
The second type of color copying machine is much faster in operation but also much more expensive to manufacture. Such a copying machine comprises three or four photoconductive drums or belts. The original document is passed over all of the drums in one scanning movement, sequentially imaging the drums through three respective primary color filters. A toner development unit is associated with each drum. The copy sheet is fed through the machine in one pass, with the toner images being transferred thereto in register through sequential engagement with the drums.
In addition to the increased cost of the three or four drums compared to only one drum or belt in the first type of color copying machine, an intricate mechanism is also required in the second type of copying machine to ensure perfect register of the three of four toner images on the copy sheet.
A full color copying machine is unnecessary in many business operations where only commercial documents are copied, since such documents generally only comprise the colors black and red, in addition to a white background. This is because accounting records and the like generally contain credit entries in black and debit entries in red. Since in many such documents the debit and credit entries may be distinguished from each other only by the color of ink, many offices have purchased or leased full color copying machines for copying such records. The full color copying capability is wasted since it is only necessary to distinguish red from black on the copies.
The basic process of the above indicated patent application overcomes the drawbacks of the prior art by providing a simple and low cost copying machine which can produce copies in two colors, such as red and black, using only one imaging operation and comprising only one photoconductive drum or belt. In the basic process, a photoconductive material comprises a conductive substrate, an inner photoconductive layer formed on the substrate and being sensitive to visible light and an outer photoconductive layer formed on the inner layer which is insensitive to red light. An electrostatic charge is applied to the outer layer while radiating the material with light to make only the inner layer conduct. Then, an electrostatic charge of the opposite polarity is applied to the outer layer in the dark. A light image of an original document is radiated onto the outer layer, white areas of the image causing photoconduction of both layers and red areas thereof causing photoconduction of only the inner layer. As a result, white areas of the material have zero surface potential while red and black areas have non-zero surface potentials of opposite respective polarities. Red and black toner particles of opposite electrostatic charge are applied to the material and adhere to the respective charged areas to form a red and black toner image which is transferred to a copy sheet.
A problem with the prior process is that the electrostatic image corresponding to the black image areas is predominated by the charge at the interface of the inner and outer layers whereas the electrostatic image corresponding to the red image areas is predominated by the charge at the surface of the outer layer. Thus, the resolution of the red image areas is higher than that of the black image areas. This is undesirable since most of the printing on business documents and the like is black rather than red, and the color which is used most should have the highest possible resolution rather than vice-versa.